CSLA M16s with underbarrel grenade launchers can't be fitted with suppressors.

In the case of the M16 Carbine/GL, you could say that the GL support brace on the barrel obstructs the suppressor, but I'm pretty sure M16s can be fitted with suppressors that don't go all the way back to the front sight post.

In the case of the M16A2/GL, there is no obstruction, but it still doesn't accept suppressors.